Heretofore, in an image formation device adopting electrophotography, an electrophotographic photoconductor as an image carrier is electrified by an electrifier, a latent image is formed by radiating a beam according to image information on the photoconductor, the latent image is developed by a developing machine and the acquired image is transferred on sheet material and an image is formed.
In the meantime, as color printing is demanded, a tandem multiple image formation device wherein plural image carriers for each image formation process are provided, a cyan image, a magenta image, a yellow image, desirably a black image are formed by the respective image carriers and a full color image is formed by overlapping and transferring each color image on sheet material in a transfer position of each image carrier is also proposed.
Such a tandem multiple image formation device is advantageous in speedup because each image formation part is provided every color.
However, there is a problem that it is difficult to satisfactorily register each image formed by different image formation parts. The reason is that the misregistration of four color image formation positions transferred on sheet material finally appears as a irregular color or as the change of a color tone.
For the types of the above misregistration of transfer positions, as shown in FIGS. 5(a), 5(b), 5(c) and 5(d), there are misregistration (a top margin) in a write direction (shown by an arrow A in FIG. 5(a)) on a scanning line on a transfer material 9, misregistration (a left margin) in a scanning direction (shown by an arrow B in FIG. 5(b) perpendicular to the direction shown by the arrow A), misregistration in a diagonal direction shown in FIG. 5(c) and misregistration due to an error of magnification shown in FIG. 5(d) and actually, an image in which the above four types of misregistration is overlapped appears.
The above misregistration is mainly caused by the lag of timing at which each image formation station begin to draw an image in the case of a top margin shown in FIG. 5(a) and by the lag of timing at which each image formation station draws an image in the case of a left margin shown in FIG. 5(b), that is, the lag of timing for the start of scanning on one scanning line.
The misregistration in a tilt in the diagonal direction shown in FIG. 5(c) is caused by misregistration in an angle at which a scanning optical system is arranged or misregistration in the angle of the rotation shaft of a photoconductor drum, and the misregistration due to an error of magnification shown in FIG. 5(d) is caused by difference in the length of a scanning line due to the error of optical path length from the scanning optical system of each image formation station to a photoconductor drum.
To eliminate the above four types of misregistration, first, the quantity of misregistration is corrected by adjusting the scanning timing of each color as to a top margin shown in FIG. 5(a) and a left margin shown in FIG. 5(b). As to the misregistration due to an error of magnification and the misregistration in a tilt, the quantity of misregistration can be corrected by independently adjusting a pair of mirrors 101 and 102 the respective mirror faces of which are held opposite at a right angle out of three folded mirrors 101, 102 and 105 arranged on the way of the optical path of each station shown in FIG. 6 in directions shown by arrows M and N with the body of the device as shown in FIG. 6 by actuators 103 and 104.
For the actuator for the above adjustment, a linear step actuator provided with a step motor which is a driving source for enabling gradual linear movement and others are used.
The above configuration is extremely effective to prevent the misregistration of four colors.
However, according to the above prior configuration, as the configuration of correction means for the misregistration due to an error of magnification shown in FIG. 5(c) and the misregistration in a tilt shown in FIG. 5(d) is particularly complicated and many parts are required for each correction means, the above prior configuration has a defect that the above correction means is readily vibrated. Therefore, there is a problem that a photoconductor is vibrated in the position of a laser beam radiated on the photoconductor, it appears as the nonuniformity of scanning, the nonuniformity of scanning appears as difference in a color tone of an output image and the image is remarkably deteriorated.
The above problem is a very serious problem when an image free of the misregistration of colors and having high quality is to be formed by a digital multicolor image formation device.
The object of the present invention is to provide a multiple image formation device wherein the configuration of misregistration correction means is simplified, the cost is reduced, further, adjustment for misregistration is facilitated and misregistration in each pixel in case images respectively acquired by optical scanning using plural light beams are overlapped is eliminated.